In recent years, as a variety of electronic equipments become compact, electronic devices to be installed inside the electronic equipments have become more compact and higher in performance. As one of the electronic devices, there is a ceramic electronic device, such as a CR built-in substrate and a multilayer ceramic capacitor, and the ceramic electronic devices have been required to be more compact and higher in performance.
To pursue a more compact ceramic electronic device having a higher capacity, there is a strong demand for making a dielectric layer thinner. Recently, a thickness of a dielectric green sheet composing a dielectric layer has become several μm or less.
To produce a ceramic green sheet, normally, a ceramic coating material composed of ceramic powder, a binder (an acrylic based resin and a butyral based resin, etc.), a plasticizer (phthalate esters, glycols, adipic acids, and phosphoric esters) and an organic solvent (toluene, MEK and acetone, etc.). Next, the ceramic coating material coated on a carrier sheet (a supporting body made by PET and PP) by using the doctor blade method, etc. and dried by heating.
Also, a method of producing by preparing a ceramic suspension wherein the ceramic powder and binder are mixed in a solvent, then, extruding a film-shaped molded item obtained by molding the suspension by twin-screw has been considered in recent years.
A method of producing a multilayer ceramic capacitor by using the ceramic green sheet explained above will be explained in detail. An internal electrode conductive paste containing metal powder and a binder is printed to be a predetermined pattern on the ceramic green sheet and dried to form an internal electrode pattern. After that, the green sheet is peeled from the carrier sheet and stacked by a predetermined number of layers. Here, two methods are proposed, that are a method of peeling the green sheet from the carrier sheet before stacking in layers and a method of peeling the carrier sheet after stacking in layers and adhering by compression, but the difference is not large. Finally, the stacked body is cut to be chips, so that green chips are prepared. After firing the green chips, external electrodes are formed, so that a multilayer ceramic capacitor and other electronic devices are produced.
When producing a multilayer ceramic capacitor, an interlayer thickness of sheets formed with internal electrodes is in a range of 3 μm to 100 μm or so based on a desired capacitance required as a capacitor. Also, in a multilayer ceramic capacitor, a part not formed with internal electrodes is formed on an outer part in the stacking direction of the capacitor chip.
Generally, when a thickness of the green sheet becomes thin, there arise problems that smoothness on the surface of the sheet declines and stacking becomes difficult.
In recent years, as electronic equipments become more compact, electronic devices to be used therein have rapidly become more compact. In multilayer electronic devices as typified by a multilayer ceramic capacitor, rapid development has been made on increasing the number of layers to be stacked and attaining a thinner interlayer thickness. To respond to the technical trends, a thickness of a green sheet, which determines the interlayer thickness, has almost become 3 μm or less to 2 μm or less. Therefore, in a production process of a multilayer ceramic capacitor, it is necessary to handle extremely thin green sheets and to design very advanced green sheet properties.
As characteristics required as the properties of such an extremely thin green sheet, sheet strength, flexibility, smoothness, adhesiveness when being stacked, handlability (electrostatic property), etc. may be mentioned, and balance of a higher order is required.
Particularly, when the sheet becomes thin, roughness (unevenness) on the sheet surface cannot be ignored with respect to the thickness. Namely, there appears a portion where the sheet is thin. It is anticipated that this portion has susceptibility to a voltage application at firing and causes short-circuiting. Therefore, to produce a sheet having a unified thickness and smooth surface is essential element technique in producing a multilayer chip capacitor.
Note that, as described in the Japanese Unexamined Patent Publication No. 6-206756, there is known a technique of using a polyvinyl butyral resin having a polymerization degree of 1000 or more as a binder in green sheet slurry containing an aqueous solvent for a purpose of eliminating a short-circuiting defect.
However, the Japanese Unexamined Patent Publication No. 6-206756 is not for particularly attaining a thinner organic solvent based green sheet, and also has the problem that surface smoothness declines and stacking becomes difficult when a thickness of the green sheet is made thin.
Also, the Patent Publication No. 2866137 discloses a technique of using a solvent having a high evaporation rate to improve a surface property of the sheet by making the evaporation rate high. However, although the method of making the evaporation rate high is effective to obtain a thick sheet, it gives an adverse effect of deteriorating the surface property in the case of making the sheet thin.
Also, as disclosed in the Japanese Unexamined Patent Publication No. 2000-335971, there is known an invention of regulating a blending composition ratio, a defoaming condition and a drying temperature condition of the sheet by an aqueous coating material.
However, in this technique, a sheet having a desired property may not be able to be obtained due to the limited composition and the procedure becomes cumbersome and complicated because a defoaming step is added.
Furthermore, as disclosed in the Japanese Unexamined Patent Publication No. 2001-114568, there is also known a technique of improving a surface property by extending the sheet by applying pressure.
However, in this technique, the sheet may be damaged due to the applied pressure when the green sheet is made thin.